What I'm looking for is "What rear-end ratio is going to give me good 'get-up-and-go', but still allow me to drive at 70 MPH on the freeway, and not let the engine sustain high RPM's for extended periods of time"??

I've got a 3.25 now. I was thinking of going with a 3.00, but now I'm thinking of going with a 3.50 (the "calculator" shows me at 3300 RPM at 70 MPH with the 3.50, which doesn't seem too detrimental for sustained periods of time).

What do you guys think?? (I know I've asked this before) But . . . 3.00 gears?, or 3.5 gears?, or stay with the 3.25?? Anyone with more experience with rear end gears than me?? (I have none :p)

Thanks,

Rick

__________________
'67 390 GT Coupehttp://a7.cpimg.com/image/89/A1/2699...-016800A8-.jpg
[color:"red"]*Click on the pic for more info about the car and pictures on my website!*[/color]"Yesterday, the bottom fell out of my world, so I drank 8 pints of Felinfoel Ale and this morning, the world fell out of my bottom." -Haydn Croft

Rick
It depends on how much highway driving your planning on doing and what you concider to be high rpms. 3300 will kill the gas mph but give you good get up and go. If thats not a concideration on the gas mileage then go with the 3:50s you'll love them.

Gas milage is not an issue at all . . . All I'm looking for is to be able to cruise on the freeway for, perhaps, hours at a time at a sustained engine RPM, that's not going to be detrimental to my motor, and still get all the 'Umph" I can out of the ol' 390.

So, I guess what I'm asking is: Is cruising at somewhere between 3,300 and 3,500 RPM's for long periods of time going to harm my engine??

Rick

__________________
'67 390 GT Coupehttp://a7.cpimg.com/image/89/A1/2699...-016800A8-.jpg
[color:"red"]*Click on the pic for more info about the car and pictures on my website!*[/color]"Yesterday, the bottom fell out of my world, so I drank 8 pints of Felinfoel Ale and this morning, the world fell out of my bottom." -Haydn Croft

Personally I would go with the 3.50 and go to a 26.1 inch tire suck as a 235/60r15 would be. It would drop your rpm to around 31 at 70 and if you wanted to go with some 255/60r15 which is a 27 inch tire it would further drop em down to a more than tolerable 3000 rpm which I have driven that in mine b4 I put the overdrive in with 2.70 gears in it for longer than 3 hours at a time and not hurt anything. Not even run hot. But I was going a little faster than 70. I actually have a set of 3.50 gears that will be here on friday and hopefully I will have them in soon after and let you know. More so the swap to an AOD is so simple it took me a weekend to do it and get all the parts to change out that if you wanted to go steeper than 3.50 think about puttin an AOD in there. Just my .02 ~Heath

The ol' 390 has been rebuilt, probably less than 15K miles ago (by the PO). Of course, it's torquey from idle up, but has a fairly narrow HP range from 3200 RPM to 5800 RPM (yeah, it really craps out around 5800 :p)

Doesn't burn oil at all . . . it's just that, jeez . . . at 3200 RPMS, it sounds as if it's 'winding-out', even though the HP is just starting to 'kick-in', and so, I guess I've just got to learn that, even though it sounds like it's reving high, doesn't mean it actually is (relatively speaking).

Rick

__________________
'67 390 GT Coupehttp://a7.cpimg.com/image/89/A1/2699...-016800A8-.jpg
[color:"red"]*Click on the pic for more info about the car and pictures on my website!*[/color]"Yesterday, the bottom fell out of my world, so I drank 8 pints of Felinfoel Ale and this morning, the world fell out of my bottom." -Haydn Croft

If you only want to do it once, order ~3.70-3.89 gears and swap in a 5 speed OD trans. Everything else will be a compromise. 3.00 or 3.25 (or even 3.50) gears have little value in terms of performance. The total cost for a conversion is reasonably low, and complete kits are available. The 5 spd trans in my '66 has absolutely transformed the way the car drives. Blast through the bottom 4 gears, then shift to 5th for 60mph @ 2000 rpm. There is NOTHING that you can do to your car for the $ that will make that much difference in performance and drivability.

Assumming my tach is accurate (Sunpro), with my 3.50 gears (9") and 225/60R15's, I'm turning about 3.2k at around 75mph in Overdrive. I can't image you're really turning 2k at 60mph with ~3.70-.389 gears, even with an overdrive.

Before I put the 3.50's in back I would turn 2.2k at 60mph with the 2.79 gears in overdrive.

I would also have to disagree that 3.50 gears have no value in terms of performance. No, I'm not going to beat a similar engine with 4.11's in it, but I have run a 14.33, with my old, way too lean carb, and the track lock not working (spun the tire through the first half of 1st gear). Now with my richer carb, and soon as I get the track lock fixed, I'm sure I'll be in the 13's, which, IMHO, is not to shabby for a daily driver.

__________________VMF Jedi Knight who's spirit will be with us forever.

I am using 3.55:1 rear gears in Lottie, and at 75 miles per hour, she is turning right around 3,200 or 3,300 RPM.

As far as being detrimental? As long as you drive it at steady pace (as in not flooring the pedal and then returning to normal speed again and again) then you should be OK. I've been driving Lottie on the freeway at 75 to 80 miles per hour for over 10,000 miles now and it's been fine.

Remember, these motors are often put into boats with few bottom end modifications, and boats only run in one gear... Sometimes up to 4,000 RPM for sustained periods.

I know what you 're saying about the way the motor sounds at 3,200 RPM though. Remember, some of that sound you're hearing is your mean, big-block exhaust too.

I've got a small block in mine, but with a 4.11, I am driving at around 3400RPM at 60MPH. No problems yet (~50K on this rear-end with a lot of "easy" 65MPH highway miles), but I do burn oil... the 1970 302 has unknown miles, but was put in about 100k ago. Car total is 430k miles. :: I am planning to put in a 5spd though as I don't like to rev for long periods. If you stick with the 4spd, I'd not go over 3.55, though.
Daniel

Crapping out around 5800 doesn't sound too bad for a 390, IMO. That'll put your top end around 115-120mph with the 3.55s. The best cure for that 'winding out' sound you're hearing is to turn up the stereo. ::

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